Not a sound will be heard out of this! (7)
I believe the answer is:
earshot
'heard out of this' is the definition.
The definition suggests a singular noun which matches the answer.
'not a sound' is the wordplay.
'not a' becomes 'ear' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sound' becomes 'shot' (I have seen 'Sound of gun' mean 'shot' so perhaps 'sound' could also mean 'shot').
'ear'+'shot'='EARSHOT'
'will be' is the link.
Can you help me to learn more?
(Other definitions for earshot that I've seen before include "catching distance?" , "Extent of being heard" , "Range over which one may hear or be heard" , "Range of audibility" , "Range for hearing" .)